IMO a young somewhat MLB experienced LHP like Santiago has more value than a Of prospect like Eaton.

As far as what the Sox should have received, It depends on the quality of the return. I think we should've got two prospects (because that's what Eaton still is) another mid-level pitching prospect would've sufficed.

I think you have unrealistic expectations or you are overvaluing Santiago. This was basically a prospect for prospect swap where the Sox traded the only strength they have to acquire a player in a position of need that has a huge ceiling. The Sox need players like Eaton. This is the price this team is paying for years of bad moves and bad drafting.